The present invention relates to a matrix display device comprising a plurality of row and column address conductors, a plurality of electro-optical display elements each of which is connected between a row conductor and a column conductor in series with a two terminal non-linear resistance device which comprises first and second parallel conduction branches capable of allowing current flow in respective opposite directions with each branch exhibiting a threshold characteristic and comprising a plurality of series-connected diode elements.
An active matrix display device of this kind is suitable for displaying alpha-numeric or video, e.g. TV, information.
Matrix display devices of the above kind using diode elements and in which the electro-optical display elements comprise liquid crystal display elements are well known. In these known forms of liquid crystal display devices, for example as described in GB-A-2,129,183, each display element is connected in series with a diode ring type of bi-directional non-linear resistance device having parallel branches which each contain series-connected diode elements between a row, scanning, conductor and a column, data, conductor.
The purpose of the diode ring circuit is to act as a switch in series with the display element. When a given row of the display device is to be addressed the voltage applied to the row conductor concerned is taken to one of two predetermined levels. The polarity of the voltage applied across the display element is periodically inverted, usually every field, to prevent unwanted degradation of the liquid crystal material and generally poor image quality.
During the "select" period the voltage across a diode ring circuit and its associated display element (and other diode rings and display elements in the same row) causes the diode ring to operate in the charging part of its circuit characteristic. In this region the diode ring circuit current is large and the display element capacitance rapidly charges to a voltage, depending on the voltage applied to the column conductor at that time, the select voltage applied to the row conductor, and the forward voltage drop across the diode ring circuit at the end of the charging period. At the end of the select period the row voltage falls to a new, lower, and constant value which is selected so that the mean voltage across the diode ring circuit during the subsequent interval until its assciated display element is next required to be addressed, usually in the succeeding field period for TV display, is minimized. In theory, assuming an ideal situation, this sustain or hold voltage is equal to the mean of the rms saturation and threshold voltages.
Under these conditions the maximum voltage of either polarity appearing across the diode ring circuit is equal to the peak to peak voltage on the column conductor, which in turn is equal to the difference between Vsat and Vth. As the voltage across the diode ring circuit increases larger leakage currents flow through the diodes and vertical crosstalk can appear. For a given level of display performance it is possible to derive a maximum acceptable diode voltage which can be controlled by varying the number of diode elements for each display element. Thus the diode ring circuit characteristics can be matched to the liquid crystal by suitable choice of the number of diode elements used.
Clearly, the smaller the difference between Vsat and Vth, the fewer diode elements are needed. However, a certain difference is needed to allow grey scale levels to be accurately reproduced. As the number of diode elements increase, so does the chance of producing a faulty diode element. Also, for a display device operated in transmission mode, and bearing in mind that the diode elements are usually fabricated side by side and situated laterally adjacent an electrode of their associated display element on a substrate of the display device, the effective optical transmission area of the display device becomes smaller as more diode elements are used, thereby producing a dimmer display for a given backlight power.
The effect of one diode element in the circuit becoming open circuit is that the diode ring circuit cannot conduct for one of the polarities of drive voltages and in this situation the voltage across the display element then has a different rms value from the correct one. The voltage also has a large 25 Hz component as well as a large dc component. The combined effect of these characteristics is that the display element has a different brightness, exhibits a 25 Hz flicker, and does not respond to changes in drive signal in the same way as other display elements. Such a display element is therefore seen as a visible defect in the display picture and renders the display device unacceptable for high quality display purposes. In order to alleviate this problem, it has been proposed in GB-A-2129183 mentioned earlier that two diode ring circuits be used for each display element so that, in the event of one diode ring circuit proving defective through an open-circuit diode element, the other diode ring circuit controls the display element as required to provide satisfactory operation. This proposal suffers from the disadvantage that the required duplication of diode rings complicates manufacture and leads to an even greater proportion of the display device's area being occupied by the diode rings and therefore still less active display area.
It is an object of the present invention to provide an improved matrix display device of the kind described in the opening paragraph.
More particularly it is an object of the present invention to provide a matrix display device using diode ring circuits as display element switching devices which requires fewer diode elements to be used than usual while retaining substantially the same or similar operational performance and thereby reducing the chances of a defective diode element occuring.